Industrial and utility boilers are used to burn fossil fuel for the purpose of generating heat and/or electricity. In the process of converting the fuel into energy, a waste gas stream of significant volume is generated. This waste gas stream may contain contaminants that must be removed before being emitted into the atmosphere, and these contaminants include, but are not limited to, sulfur dioxide (SO2), carbon dioxide (CO2), carbon monoxide (CO), and vapors of metal compounds such as mercury, selenium, arsenic, chromium, and the like. To accomplish removal of these contaminants, operators of such processes will install equipment to capture the contaminants and add reagents to react with them to form less harmful substances; one of these processes is wet flue gas desulfurization (FGD). Wet FGD systems typically have a slurry reactor/absorber section and a solids dewatering section.
In the absorber section, circulating slurry is sprayed into the absorber which neutralizes the acidic gasses produced by the coal-fired boiler with alkaline slurry or liquor of compounds such as lime, limestone, sodium hydroxide, magnesium hydroxide or sodium sulfite. When the alkaline slurry or liquor reacts with the major acid gas, SO2, in the absorber, it is converted to sulfite, and a slurry rich in sulfite is produced. The slurry is collected in a reaction tank where additional alkaline reagent, such as lime, limestone, or sodium hydroxide, is added, and in the case of forced oxidation scrubbers, air is forced to bubble through the slurry oxidizing the sulfite to sulfate.
In the dewatering section, limestone forced oxidation (LSFO) scrubbers form calcium sulfate or gypsum (CaSO4.2H2O) byproduct which may be dewatered, filtered and sold for beneficial use by others such as in the production of wallboard. Alternatively, limestone inhibited oxidation (LSIO) scrubbers may be used in which an inhibitor, such as emulsified sulfur, thiosulfate, or iodide, is added to the process to reduce the formation of sulfate so that a mixed product of calcium sulfite and calcium sulfate (CaSO3/CaSO4.1/2H2O) is formed as the byproduct.
One significant disadvantage of the process used in lime-based or magnesium-enhanced lime-based scrubbers is associated with the supply and supply logistics of the lime reagent, such as availability, shipping concerns, proximity of supply, and cost. Those issues have created a significant concern on the part of some owners and operators of lime-based scrubbers in the industry. This has motivated many owners and operators to convert lime-based scrubbers to limestone based scrubbers. However, while such a conversion may seem straightforward, it is not.
The equipment that is used in a lime-based process differs from the equipment required for a limestone-based process, and converting from lime to limestone historically has been thought to require significant modification and replacement of the equipment by the owner/operator. There is a need, therefore, for a method of converting lime-based scrubbers to limestone-based scrubbers by maximizing the reuse of the existing equipment in the lime-based process, adding a limited amount of additional equipment, and operating the converted unit in a manner that is unique to currently operating limestone units.